I've been doing gold and silver swaps for about a year now- but only using physical metals. It makes a pretty decent profit. The ratios interchange on a pretty infrequent basis though.
I've considered using the same strategy trading online but my current broker doesn't support gold and silver transactions. It would be even more profitable though, as transaction costs (shipping etc.) would be eliminated, however, because the ratios take awhile to turn, it would require buying and holding positions for between 3 and 6 months at a time and if I'm not mistaken swaps [interest on the trades] might get hefty.
In all though, in my opinion a person can't go wrong buying and holding precious metals right now- with gold and silver swaps it becomes even more lucrative.